How does social support interact with intrinsic capacity to affect the trajectory of functional ability among older adults? Findings of a population-based longitudinal study

•Social support interacted with intrinsic capacity to affect the downward trajectory of functional ability among older adults.•The interaction effects varied with the types of social support.•Functional ability significantly declined over 3 years among older adults. The ecological model of health and ageing has proposed that functional ability (FA) is determined by the interaction between intrinsic capacity (IC) and environmental characteristics. This study empirically examined how social support, as an important social environmental resource, interacts with IC to affect FA trajectories among older adults. This was a prospective three-wave cohort study with a sample of 775 community-dwelling older adults. Social support, IC and FA were assessed using the Social Support Rating Scale, the revised Integrated Care for Older People screening tool and the Lawton Instrumental Activities of Daily Living Scale, respectively. Latent growth curve models (LGCM) were implemented to test their relationships. FA significantly declined over 3 years, and the detrimental effect of impaired IC on the deterioration rate of FA was buffered by subjective support but was aggravated by support utilization and was not changed by objective support. FA decline among older adults with impaired IC was observed in those with low subjective support or with high support utilization but not in those with high subjective support or with low support utilization. Among older adults with intact IC, FA decline was observed in those with low support utilization but not in those with high support utilization or with low or high subjective support. Subjective support may prevent FA decline among older adults with impaired IC, while support utilization may benefit older adults with intact IC but may be detrimental for those with impaired IC. Social support interventions to optimize FA trajectories should improve older adults' perceptions of support and bridge the gap in support utilization among older adults with impaired IC.